Thousands of people fled from Syria into Turkey on Wednesday as moderate rebels and Kurdish forces fought Islamic State insurgents holding the Syrian border town of Tel Abyad.

A Reuters photographer at the scene said the refugees had entered Turkey through a makeshift border crossing overseen by Turkish gendarmerie officers, and that many of them were women and children.

A Turkish official said 2,000 refugees were being registered on Wednesday after more than 6,800 were admitted in the area last week.

He said they were fleeing advances by Kurdish YPG forces as well as aerial bombardment by the United States and Arab allies trying to help the Kurds push back Islamic State.

The northeastern corner of Syria is important to the radical Islamist group because it links areas under Islamic State control in Syria and Iraq.

The group last week launched an offensive on the provincial capital, the city of Hasaka, which is divided into zones run separately by the government of President Bashar al-Assad and a Kurdish administration.

But Syria's Kurds have also sought to take advantage of Syria's complex war to expand their control over a region, stretching from Kobani to Qamishli, that they see as part of a future Kurdish state.

Turkey, for its part, fears that this will encourage separatism in its own, adjacent Kurdish region.

The Turkish official said it appeared that all the refugees were Syrian or Iraqi Arabs, rather than Kurds.

"A significant demographic change is taking place in the area. Arabs are being pushed away as Kurds flow in," he said. "Moving forward, the native population of the region might not have a place to go back to."